1. Field of the Invention
The present invention relates to a method for the additional transmission of information via a digital auxiliary channel such as is known, for example, from the article "Optical Line Codes Bearing Ancillary Channels" by K. W. Cattermole and W. D. Grover, IEE Colloquium on Data Transmission Codes, London, November, 1980. The present invention also relates to such a method as applied to an optical transmission system as described in telcom report 6 (1983), supplement "message transmission by light", pages 127-132.
2. Discussion of Background
In the field of line-conducted digital signal transmission, for example of PCM transmission in telephone channels, the use of optical wave guides is becoming more and more significant because of the large band width becoming available by this means.
In this connection, the replacement of electrical conductors by optical wave guides initially refers to the main transmission path via which the data present are transmitted from one station to the next. But in addition to the main transmission path, auxiliary and service channels are also needed for controlling and monitoring the operation of the line network and, if necessary, to pass on instructions or alarm signals. In the conventional electrical coaxial line systems, such auxiliary channels are provided by additional pairs of lines in parallel with the coaxial lines.
Transferring this principle of the separatelyconstructed auxiliary channels to the technology of optical wave guides leads either to combining an optical wave guide (main transmission path) with an electrical pair of lines, auxiliary channel) or to a plurality of optical wave guides having separate tasks.
Both solutions are costly because of the additional conductors needed. In the case of the first solution, the advantages of freedom from interference and of direct-current isolation of optical fibers are lost due to the use of the electrical pair of conductors. In the case of the second solution, channel capacity is wasted because only a fraction of the high transmission capacity of an optical wave guide is utilized due to the low signal rate in the service channel.
For these reasons, people have searched for some time for solutions in which, by taking special measures, the same optical wave guide can be used both as the main transmission path and as the auxiliary channel. Of these solutions, those are of particular interest in which the auxiliary channel can be used without interrupting the main transmission path. Thus, for example, it has been proposed to implement an independent auxiliary channel by transmitting the corresponding information by modulating the clock frequency (Electronics Letters 16 (16), July, 1980, pages 624-626).
Another proposal relates to analog transmission by amplitude or frequency modulation within a frequency range below the traffic band (Philips Telecommunication Review 40 (2), July, 1982, pages 79-86).
Such "analog" auxiliary channels, however, require elaborate signal processing (modulation, demodulation), necessitate changes and interventions at the optical transmitter and receiver and have a disadvantageous influence on the transmission quality in the main transmission path.
For these reasons, it has been proposed in the article by K. W. Cattermole and W. D. Grover initially mentioned, to utilize the redundancy of the line code, used in optical data transmission, for transmitting information in binary form by modifying selected bit patterns of the data stream in the main transmission path, in which arrangement one information unit is represented by the presence or absence of a modification ("digital" auxiliary channel).
As selected bit patterns (SOP: Signalling Opportunity Pattern), bit patterns are used which occur with sufficient frequency in the present data stream and can be converted in simple manner into modified bit patterns which do not occur in the original data stream and can thus be easily detected at the receiver side.
Starting with a transmission system at the electric interfaces of which the data are present in a pseudo-ternary bipolar code and in which these data are converted into the redundant, binary line code 2 AMI (Alternate Mark Inversion) for optical transmission, Cattermole and Grover specify as selected bit patterns (SOP) the sequences 1011 and 0100 which are associated with the sequences 1111 and 0000 as modified patterns.
The utility of the bit patterns specified is restricted to transmission systems in which the special codes are used which have been assumed.
A different optical wave guide system having a transmission rate of about 2 and 8 Mbits/s, as described in the printed supplement "message transmission by light", pages 127-132 of the telcom report 6 (1983), is based on interface signals which are coded in so-called HDB-3 code (High Density Bipolar) according to CCITT Recommendation G 703, Annex A, and are converted into the MCMI code (Modified Coded Mark Inversion) for optical transmission.